Background: Maintaining balance is essential for older adults to preserve independence and reduce fall risk. However, empirical evidence linking trunk stability with balance and gait is scarce, partly due to the lack of accessible field tests. Smartphone-based accelerometry field tests offer a promising approach to assess trunk stability outside the laboratory. Objective: The study aimed to determine (1) the association between trunk stability and whole-body balance and gait and (2) the extent to which trunk stability and whole-body balance field tests align with laboratory-based assessments in older adults. Methods: A total of 53 physically active older adults performed 2 trunk stability tests (ie, the lumbopelvic stability [field] and the unstable sitting posturographic [laboratory] tests), 2 whole-body balance tests (ie, the tandem stance posturographic [laboratory] and the whole-body static balance [field] tests), and 2 gait-related tests (ie, the straight-line gait and the modified Timed Up and Go [field] tests) 2 times. A Spearman correlation analysis was performed after assessing the reliability of the variables. Results: Significant positive low-to-high correlations were found between the lumbopelvic stability and the whole-body static balance tests (0.380≤≤0.700;